The more Attorney General Eric Holder stonewalls investigators looking into the Fast and Furious gunrunning scandal, the worse it will get for him, top Republican Sen. Charles Grassley warned in an exclusive Newmax.TV interview.

“I always tell every agency… that I have been investigating, the longer you stonewall, when the truth comes out, the more egg you are going to have on your face,” the ranking member of the Senate Judiciary Committee said.

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Grassley accused Holder of refusing to turn over documents that would show how much he knew about the project that allowed hundreds of assault weapons to fall into the hands of Mexican drug cartel leaders. He again urged him to cooperate fully.

“If he’s got any questions about what he meant to say or not say, cooperate with our investigation, give us all the documents that we ask for and let those documents speak for themselves,” Grassley said.

The Iowa senator was speaking as the whole Fast and Furious row threatened to blow up in Holder’s face, potentially becoming a major election issue next year for President Barack Obama, who gave his unequivocal backing to his attorney general minutes before the interview.

Under Fast and Furious, at least 1,400 assault weapons were allowed to cross the border into Mexico in the hope that they could be traced and would lead agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to the cartel bosses.

But most of the weapons soon disappeared and they have been linked to dozens of crimes south of the border, including the February murder of U.S. immigration officer Jaime Zapata who was working from the U.S. Embassy in Mexico City.

Some of the guns were also used in this country. Two were involved in the December murder of border agent Brian Terry in Arizona; 40 were found in a stash in El Paso, Texas in January; and now Republicans say at least 11 crimes have been committed on U.S. soil with Fast and Furious weapons.

Holder claimed in evidence to the House Judiciary Committee in May that he had only learned of the project “a few weeks ago.” But newly released emails show that Michael Walther, the director of the National Drug Intelligence Center told him about it in at least five weekly memos, starting on July 5 last year.

Already the scandal has cost the jobs of several ATF officers, including acting head Kenneth Melson, who was reassigned in August.\

Grassley said he personally handed letters to Holder in January asking for details of the scheme and expected full cooperation. But when he did not receive it he was forced to turn to California Rep. Darrell Issa, chairman of the House Oversight Committee.

“I thought I would get full cooperation and I could take care of it myself, but when I got stonewalled, I went to Rep. Issa and said, ‘Could you help us?’

“He started digging into it and, boy, has he been a big help. He has subpoena power because he is in the majority in the House of Representatives and what I couldn’t get, he subpoenaed and has helped the process along.

“But we have not still had the full cooperation of the Justice Department.”

Grassley was diplomatic when it came to suggestions that Obama should appoint a special prosecutor to investigate Holder’s role in the scheme. He said he is not calling for such a move at this time, while also saying that if the president wanted to live up to his campaign promise of increased government transparency he would appoint one and that he would not discourage others such as Rep. Lamar Smith of Texas, the chairman of the House Judiciary Committee from making such a call.

“I am not sure at this point that I would ask for one but I am surely not going to discourage Chairman Smith for asking for one,” he said. “But I want to make it very clear, special prosecutor or not, I’m going to continue my investigation.”

Other GOP members have gone even further than Smith with both Rep. Blake Farenthold of Texas and Rep. Raul Labrador of Idaho calling on Holder to resign now.

“A lot of people tell you that there are a lot of reasons why Holder ought to be fired other than just Fast and Furious, but right now I’m going to limit my comments to having more certainty of personal involvement,” said Grassley. “The more we investigate the more we know that he knew about it or at least should have known about it.

“But I am after who OK’d this and then I would say, if it’s Eric Holder, then he ought to resign. But I am not going to say that Eric Holder ought to resign until I know who the person was that approved this.”

Grassley also said one of the main objectives of his investigation was to make sure a “stupid” project like Fast and Furious is never authorized again.